Very fine fibres produced from polymer solutions, often referred to as nanofibres, are useful in a wide variety of applications, including filter media, tissue-engineering scaffold structures and devices, nanofibre-reinforced composite materials, sensors, electrodes for batteries and fuel cells, catalyst support materials, wiping cloths, absorbent pads, post-operative adhesion preventative agents, smart-textiles as well as in artificial cashmere and artificial leather.
Electrostatic spinning of fibres was, it appears, first described in U.S. Pat. No. 692,631. In principle, a droplet of polymer solution or melt is placed in a strong electric field giving rise to the repulsion between the induced like-charges in the droplet competing with the surface tension of the liquid. When a sufficiently strong electric field is applied (typically 0.5-4 kV/cm), the electrostatic forces can overcome the surface tension of the fluid and a jet of polymer solution or melt is ejected from the droplet.
Electrostatic instability leads to rapid, chaotic whipping of the jet, leading, in turn, to fast evaporation of any solvent as well as a stretching and thinning of the polymer fibre that is left behind. The formed fibres are then collected on a counter electrode, typically in the form of a nonwoven web. The collected fibres are usually quite uniform and can have fibre diameters of several micrometers, down to as low as 5 nm.
The technical barriers to manufacturing large quantities of nanofibres by electrospinning include low production rates and the fact that most polymers are spun from solution.
One general method of production utilises multiple passages such as may be provided by multiple needles. On average, solution based electrospinning, using needle spinnerets, have solution throughput rates on the order of 1 ml per hour per needle. Fibres with diameters in the range of 50 to 100 nm are typically spun from solutions with relatively low concentrations, typically 5-10 wt % depending on polymer type and molecular weight. This means that, assuming a polymer density of around 1 g/ml, the typical solids throughput rate of a needle-based electrospinning process is 0.05 g to 0.1 g of fibre per hour per needle. At this rate, production of a nanofibre web with a planar density of 80 g/m2 at a rate of 5 m2/s will require a minimum of 14,400,000 needles.
In addition, electrical field interference between the different needles limits the minimum separation between them and furthermore, continuous operation of needle-based spinnerets requires frequent cleaning of the needles as polymer deposits tend to block the spinnerets. The overall result is that the production of industrial volumes becomes almost prohibitively expensive for most commodity applications like filtration and absorbent textiles.
Formhals (U.S. Pat. No. 1,975,504) tried to increase electrospinning production rates by using a serrated wheel as the one electrode. In later designs, he used a multiple needle setup (U.S. Pat. No. 2,109,333).
Reneker et al. (international patent application publication number WO0022207) describe a process in which nanofibres are produced by feeding fibre-forming solution into an annular column, forcing a gas through the column in order to form an annular film, which is then broken up into numerous strands of fibre-forming material.
Numerous other proposals have been put forward that rely on creating jets of fibre-forming solution using needles and orifices in order to produce fibres in this manner.
A system with a significantly high throughput, known as NanoSpider, is described in international patent application publication number WO05024101. In this system the fibre forming polymer solution is contained in a dish and a partly exposed conductive cylinder is slowly rotated in it to form a thin layer of solution on its surface. A counter-electrode is placed 10-20 cm above the cylinder and hundreds of jets initiate off the surface of the cylinder and electrospin onto the target.
International patent application publication number WO 2006131081 describes a follow up type of NanoSpider technology in which the conductive cylinders are replaced by axially mounted rotatable cylindrical structures presenting multiple “discharge” surfaces from which solution is to be discharged to form the polymer fibres. The arrangement is somewhat complex and the cylindrical structures must be somewhat costly.
Japanese patent JP3918179 describes a process in which bubbles are continuously generated on the surface of a polymer solution by blowing compressed air into the solution through a porous membrane, or through a thin tube. Electrospinning jets are formed on the bubbles and fibres that form are collected on the counter-electrode. This system, it appears to applicant, requires that the bubbles in the polymer solution be formed in high volumes and that they burst very rapidly. Also, most organic solvents do not readily form foams and the given examples demonstrate spinning only with polymer solutions in water, 2-propanol and acetone. Additionally, this patent requires that the counter-electrode be placed at a suitable distance from the foam since droplets of spin solution that are created by the constantly bursting bubbles can spatter onto and harm or destroy the formed fibres on the counter-electrode.
In our pending international patent application published under number WO 2008125971 we describe an improvement of the bubble electrospinning process, based on the stabilization of the formed bubbles using a surfactant.